This invention relates to a brake system for automotive vehicles comprising devices for the electric control of the brake force distribution onto front and rear axle and/or for the control of slip. The system further comprising an auxiliary energy source and sensors to determine the wheel rotational behavior and the vehicle speed or a corresponding reference value and including electronic circuits for the conditioning, processing and logic combining of the sensor signals and for the generation of electric braking-pressure control signals.
To better adapt the apportioning of brake force onto the front and the rear axle of a vehicle to the respective axle load distribution which is subject to static and dynamic influences and therefore is dependent on the load condition of the vehicle on deceleration and other influences, load-responsive and deceleration-responsive brake force distributors or pressure reducers responsive to braking pressure according to a predetermined characteristic curve are utilized. These auxiliary means are intended to accomplish a uniform contribution of the front and rear axle to braking and to hamper locking of the rear wheels. In the event of overbraking or excessive braking pressure which results, for example, on slippery roads even at a relatively low brake pedal force, the rear wheels are constrained to lock only subsequent to the front wheels, otherwise there would be great risk of skidding, namely due to the loss of the cornering force at the locking wheel. It is a disadvantage that only a relatively rough approximation of the brake force distribution to the actual static and dynamic axle load proportioning can be attained, whereby, in order to avoid the dangerous locking of the rear wheels, the brake systems are dimensioned such that the largest share of brake force is allotted to the front wheels in the majority of situations.
Further, it has been proposed to electronically control the brake force distribution or the brake force share allotted to the rear wheels as a function of the wheel rotational behavior of the front wheels as disclosed in German patent application No. P 33 01 948.7. To this end, braking pressure modulators are inserted into the pressure fluid conduit to the rear-wheel brakes, which modulators close the pressure-fluid passage in the inactive position and which permit electronic control of the flow and hence the brake force share allotted to the rear-wheel brakes in such a manner that there results an approximately ideal brake force distribution onto the front and the rear axle. For this purpose, quick-acting, electromagnetically actuatable multidirectional control valves can be employed, for example, as braking pressure modulators which bring about the desired braking pressure rise at the rear axle by short-direction or pulsed change-over to the open-passage condition.
In another known system, a braking pressure decrease at the rear at the rear wheels (which would become necessary, for example, in the event of reduction of the friction coefficient during braking in order to prevent wheel lock) can be accomplished if additional valves are provided through which, upon an imminent locked condition, pressure fluid can be discharged to a pressure supply reservoir (German patent application No. P 33 06 611.6). This system does not provide that the braking pressure rises again in the rear-wheel brakes upon subsequent rise of the friction value during the same braking action.
It is, therefore, an object of the present invention to develop a brake system which is relatively simple and which is nevertheless reliable in all situations, because a brake force distribution can be attained during every braking action which is adapted to the respective axle load condition, and which brake system admits slip control with repeated pressure decrease and pressure increase at least at the rear axle, and at the front axle after corresponding extension.